1. Field
The disclosure of the present patent application relates to the manufacture of infrared detecting materials, and particularly to a germanium-tin (Ge-Sn) thin film for use in the manufacture of microbolometers.
2. Description of the Related Art
A microbolometer is a specific type of bolometer which is typically used as a detector in a thermal camera. Infrared radiation with wavelengths between 7.5-14 μm strikes the detector material, heating it, and thus changing its electrical resistance. This resistance change is measured and processed into electronic signals representing scene apparent temperatures which can be used to create an image.
The two most commonly used infrared (IR) radiation detecting materials in microbolometers are amorphous silicon (a-Si) and vanadium oxide (VO). Amorphous silicon works well because it can easily be integrated into a complementary metal-oxide-semiconductor (CMOS) fabrication process, is highly stable, has a fast time constant, and has a long mean time before failure. To create the layered structure and patterning, the CMOS fabrication process can be used, but it requires temperatures to stay below 200° C.
Vanadium oxide thin films may also be integrated into the CMOS fabrication process, although not as easily as a-Si, due to temperature restrictions. VO2 has low resistance but undergoes a metal-insulator phase change near 67° C. and also has a lower value of temperature coefficient of resistance (TCR), On the other hand, V2O5 exhibits high resistance and also high TCR. Many phases of VOx exist, although it appears that x≈1.8 has become the most popular for microbolometer applications. The search for semiconductors materials which are more common, and thus easier to experiment with, is ongoing.
The resistivity and the TCR of the temperature sensing layer are two main properties that influence microbolometer performance. The thin film forming the temperature sensing layer should possess a resistivity that is compatible with the accompanying read-out integrated circuit (ROIC). The resistivity value of the temperature sensing layer also has a direct effect on the electrical noise of the microbolometer. In addition, the responsivity of a microbolometer is directly proportional to the TCR of the temperature sensing layer. A temperature sensing layer with a high TCR is desirable.
The group IV Ge1-xSnx alloys have recently emerged as a promising CMOS compatible semiconductor material, and are being investigated for many photonic and microelectronic applications, Ge1-xSnx compound semiconductors have been proven to have a direct band gap and have been applied to making light emitting diodes, laser diodes, p-i-n photodetectors, photoconductors, p-MOSFETs and other devices. It would be desirable to be able to extend group IV Ge1-xSnx alloys to the manufacture of thermal sensing layers in uncooled microbolometers.
The reduction of the Ge1-xSnx alloy's band gap energy below that of germanium (Ge) alone would result in different resistivity and TCR properties when compared to just Ge. Thus, a thin film for a microbolometer and a method of making the same solving the aforementioned problems is desired.